Observations in several species have suggested certain inverse relationships between seasonal breeding cycles and seasonal cycles of hibernation. These findings prompted us to study the effects of sex steroid hormones on hibernation in the Turkish hamster. Young males with small testes were induced to hibernate by placing them in a cold room at 10 degrees C. These animals typically go through cycles of torpor 3-5 days followed by arousal for 1-2 days. During one of the arousal periods experimental animals received a subcutaneous implant of testosterone in a silastic capsule and controls received empty capsules. Hamsters which received empty capsules usually became torpid again within a few days while testosterone-implanted males almost invariably failed to reenter hibernation. This effect of testosterone was observed in both intact and castrated males. Preliminary data suggest that progesterone does not prevent hibernation and that estradiol may have a weak inhibitory effect. These results point to one potential endogenous cue for the termination of hibernation, i.e., rising levels of testosterone. Studies in the closely related Syrian hamster, and preliminary studies of our own on the Turkish hamster, indicate that testicular function is under photoperiodic control. Therefore, the testes may be a link between environmental cues and seasonal cycles of torpor. One major goal of this project is to elucidate such factors. If we are able to understand the mechanisms (hormonal, enzymatic, etc.) by which some mammals are able to hibernate, we may gain valuable information for use in cryobiological medicine. In addition, we should gain a better understanding of how metabolic enzyme systems are able to adapt to a changing environment.